Accesing content of select-options

Hi,

I need to access to the content of a field from a select-options in order to check if there is any value on it.

My idea is to have two fields in the select options, one of them, is disable at the beggining. If the user fills the first field, so the second field is enabled.

How can access to the content of field on the screen that belongs to a select options?

Thanks in advance!